Several techniques exist for producing a side weld of a tube skirt.
The first consists of positioning both side ends of the strip edge to edge, then welding them:
either by having previously beveled the edges such that they become fitted to one another asymmetrically,
or by adding tape at the seam between the edges.
This technique requires high precision regarding the edge-to-edge positioning, which is unfortunately rarely the case, since the guiding of the strip is not easy in tube skirt manufacturing devices.
The second technique consists of covering one edge with the other edge: this is a process with overlap. In this case, the positioning precision is less important, which makes the process easier. The main drawback is that it is generally necessary to avoid any decoration in the overlapping zone due to its lack of resistance to welding conditions. This results in an undecorated line of a certain width visible along the tube in the region of the weld. Additionally, the weld seam is generally clearly visible from the outside, and not aesthetically pleasing.
To resolve this problem, it is known to place a decorative layer between a sealable upper layer and an a lower support layer, so that it is practically not damaged during the side welding step. In this case, the decorative layer extends over the entire surface area of the strip. This decorative layer can also be placed upside down below the lower support layer so as to be better protected, but this technique is expensive and therefore only of interest for large quantities of tube skirts to be produced. In all cases, such a decorative layer only makes it possible to insert a colored and/or shiny and/or metallic “background” on the tube skirt.
The next step consists of performing printing on the strip so that distributors can affix their brand and various information relative to the product poured inside the tube. This step is carried out before the step for forming the sleeve and the side welding step. This is therefore an additional layer added on top of the sealable upper layer. Yet this printing layer is fragile, and the ink tends to creep under the effect of the heat added on top of the overlapping area during welding. It is therefore common practice not to print on both side edges of the strip that will form the overlapping area of the tube skirt, particularly given that many inks exist that are not weldable. The same is true for the protective varnish applied on the inks, which is not weldable.
More specifically, it is common practice to heat both the outer surface and the inner surface of the tube skirt at the overlapping area, so as to melt the material not only on the surface of the overlapping area, but also in the thickness of the overlapping area, which makes it possible to produce a robust weld that holds up over time.
Traditionally, the heating temperature T1 applied on the outer surface of the tube skirt is higher than the heating temperature T2 applied on the inner surface of the tube skirt. The goal is to melt the outer surface before the inner surface is melted, so that the layers near the inner surface, in particular including a barrier layer providing the sealing of the tube skirt, do not have time to mix and therefore create weak points in the overlapping area.
Indeed, for a traditional multilayer strip structure, if T2 was greater than T1, the inner layers would mix upon melting and destroy the strip at the overlapping area, thereby creating weak points for example resulting in a poor sealing barrier no longer guaranteeing optimal storage of the cosmetic product that will be poured inside the tube.
The aim of the present invention therefore consists of proposing a strip structure for a tube skirt that allows a side weld to be formed that is aesthetically pleasing from the outside, and robust enough to preserve the desired barrier effect. This strip structure must also allow the application of a broad ink pallet in the overlapping area of the tube skirt. The invention also aims to propose a method for producing such an aesthetically pleasing and robust side weld, as well as a device making it possible to carry out this method.